One of the operations which must normally be performed during the outage of a nuclear boiling water reactor is the decontamination of the reactor pressure vessel. This is considered to be a "critical path" operation as opposed to a "collateral" operation. In other words, it is an operation which must be performed before the succeeding operation can be undertaken and, thus, adds directly to the length of the shutdown period.
Once the head has been removed from the reactor vessel, it is essentially an open top cylindrical tank which may be on the order of twenty feet in diameter. Decontamination is achieved by spraying high pressure water on the walls of the vessel. The manner in which this is conventionally achieved is by lowering a man into the vessel in a container suspended from a crane. He then proceeds to wash down the inner wall with a high pressure hose. It is desirable to use very high pressure water for this purpose, for example, up to 10,000 psi. However, the high reaction forces acting upon the container at the end of a relatively long cable, cause the container and workman to be bounced from side to side within the reactor vessel. The man, even though wearing protective gear, is exposed to highly contaminated water droplets in a highly radioactive environment. The radioactivity, coupled with the sheer physical stress, makes this a very difficult and undesirable job.
Because of the reaction forces, it is usually necessary to reduce the water pressure below its optimum value. As a result, the decontamination period is substantially increased and may be, for example, on the order of eight hours. As reactor shutdowns are very costly, it would be desirable to shorten the time for decontamination. It would also, of course, be desirable to reduce or eliminate the exposure of personnel to the dangerous and stress inducing features of the conventional decontamination washdown.
One approach to the problem of reactor vessel decontamination is disclosed in a copending U.S. application, Ser. No. 208,690, filed Nov. 20, 1980 of Michael Vassalotti for "Decontamination Apparatus for Nuclear Reactor Pressure Vessel" which has been assigned to the same assignee as the present invention. Although that solution to the decontamination problem has proved itself highly successful, it requires the concomitant use of an equipment crane during its operation. The present invention frees the reactor crane for other work and can accomplish the same task more efficiently.